Oak Ridge National Laboratory is the world leader in providing intense neutron beams to instruments used by scientists. Bombarding materials with high concentrations of neutrons helps scientists better understand how atoms arrange themselves and move in physical and biological substances and how to design materials with improved properties for industry.
ORNL has two world-class neutron sources: the High Flux Isotope Reactor and the Spallation Neutron Source (SNS) , which is the world’s most powerful pulsed neutron source. To retain its premier status in neutron science in the 2030s, ORNL is planning to create a third neutron source by adding a second target station to complement the first one SNS has been using to make neutron beams since 2006.
In a recent talk to Friends of ORNL, Mark Champion, project director of the Proton Power Upgrade (PPU) project at the SNS, said the project will double the power capability of the SNS linear accelerator, from 1.4 to 2.8 megawatts (MW), facilitating new types of experiments and discoveries.
The upgrade will increase the beam energy by 30% and the beam current by 50%. The first target station will receive a 2 MW proton beam by the start of fiscal year 2027, possibly earlier, with the balance of the beam available for the second target station.
He noted that the upgrade will enable scientists working at the SNS “to do experiments on smaller or less concentrated samples.” Smaller samples, he added, are easier to handle in experiments in which they are subjected to extreme environmental conditions such as very high pressures, temperatures or magnetic field strengths.
Since 2021, construction in support of the PPU project at the SNS has been carried out to improve the performance of its particle accelerator and first target station, increase proton and neutron production and build a connector for sending protons to the second target station, which will be completed between 2032 and 2039 as instruments are added.
The SNS, which was shut down last August for the installation of PPU equipment, resumed neutron production on July 11 with a proton beam power of 1.7 MW.
Ahead of schedule, under budget
“Many people at SNS and elsewhere across ORNL have contributed to this success,” Champion said. “We are ahead of schedule, under budget and meeting technical objectives. We also had help from colleagues at the Department of Energy’s Jefferson Laboratory in Virginia and Fermilab in Illinois. It’s a good story.”
He explained how the SNS works while describing the components added to increase the beam capability needed to provide more neutrons for scientific experiments.
At the start, hydrogen gas is introduced into the ion source. An electron is added to each hydrogen atom (consisting of a proton and electron) to make a negatively charged hydrogen ion. Bunches of hydrogen ions are accelerated to 88% of the speed of light by radio waves created by the switching of electric fields from positive to negative at a given frequency.
The devices that convert electricity into radio-wave energy for speeding up the hydrogen ions are called klystrons, described as high-power, radiofrequency (RF) amplifiers. Electromagnets steer and focus the hydrogen ion beam along an evacuated beamline in the linear accelerator, or linac.
Champion said the linac consists of copper cavities and superconducting niobium cavities (metallic chambers that contain the electromagnetic fields) that are powered by 120 klystrons. The cavities are housed in 23 complex structures called cryomodules, which are supercooled with liquid helium.
The first one-third of the linac operates at room temperature, while the remainder uses 81 superconducting cavities cooled with liquid helium to just two degrees above absolute zero.
The linac connects to the accumulator ring, which is outfitted with magnets that steer and focus the proton beam as it circulates around the ring. As the accelerated negative hydrogen ions pass from the linac through a thin diamond foil, their electrons are stripped away so they enter the ring as high-energy protons that accumulate into bunches. An intense proton beam less than one-millionth of a second in duration is created as the protons travel around the ring about 1,000 times.
Sixty times a second short pulses of protons are delivered to a mercury target system, called the first target station (FTS). There the 1.7-megawatt proton beam (which will be increased to 2 MW) knocks out the neutrons from the mercury nuclei, a process called spallation. Pulses of neutrons are sent through neutron beam lines to neutron instruments for neutron scattering experiments and data collection on material samples.
The SNS, Champion noted, has the world’s first and only megawatt-class superconducting proton linear accelerator, and it achieved the first use of a liquid mercury target to produce high-intensity pulsed spallation neutrons at a user facility for qualified scientists at the lab or visiting from all over the world.
He said the construction of the SNS, made possible by a collaboration of six Department of Energy national laboratories starting in the 1990s, was completed in April 2006 at ORNL. The SNS has operated in recent years at a beam power of 1.4 to 1.7 megawatts on the target with availability greater than 90% against schedule and providing more than 4,000 user hours per year.
The number of scientific instruments has grown from three to 20. The second target station (STS) will deliver early science experiments in fiscal year 2033 with three initial instruments, and five additional instruments will be completed by fiscal year 2039. The goal is 5,000 user hours per year.
Champion said the PPU project, which will be completed in 2025, will provide up to 40% more power to the FTS target, delivering more neutrons to accelerate the pace of scientific discovery across a wide range of materials and technologies.
Before and during the 10 months of the SNS shutdown, he said, 28 superconducting cavities in seven cryomodules were added to the linac’s end to achieve an energy capability of 1.3 giga electron volts (91% of the speed of light); 28 high-power RF klystrons were added to power the new superconducting cavities; the existing klystrons were upgraded; three high-voltage converter modulators were added to drive the new klystrons and the existing modulators were upgraded; new magnets were added to the accumulator ring; and the FTS mercury target capabilities were increased as was the target lifetime using gas injection techniques.
In the FTS 18 tons, or 355 gallons, of mercury flows in a loop in a stainless-steel vessel. Champion listed upgrades recently made to the FTS.
When the proton beam strikes the mercury target, cavitation bubbles can damage the vessel inner wall, shortening its lifetime. Mitigation of cavitation and reduction of stress on the vessel are now achieved by injecting helium bubbles into the mercury to increase vessel life.
The interaction between the protons and mercury nuclei produces hazardous gases that are captured by a mercury off-gas treatment system, keeping the toxic isotopes out of the environment. The PPU project upgraded this system and installed a mercury overflow tank to accommodate high-flow gas injection.
A major subproject was the construction of a tunnel “stub” to connect the accumulator ring to the future second target station. The addition will enable the future tie-in of the ring-to-STS proton beam transport line without interrupting FTS operations.
The STS, Champion said, will have a solid rotating, water-cooled tungsten target. It will provide the world’s brightest “cold” neutrons, enabling studies of more complex materials. The cold neutrons are produced by moderating a neutron beam with supercritical hydrogen, reducing the neutrons’ kinetic energies and increasing their wavelengths. Cold neutrons will enable scientists to study materials with large dimensions and low-energy vibrational states, such as biological molecules and polymers.
“Bright” neutrons have been compared to bright light. Just as you find it easier to read fine print in bright light rather than in dim light, scientists need a high concentration of neutrons (i.e., an extremely high number of neutrons per second per unit area) to strike their samples so they can “see” how the sample’s atoms and molecules are organized or move during a chemical reaction or under a changing magnetic field.
A beam of pulsed neutrons from the FTS at the SNS has been compared to a flashing strobe light that provides high-speed illumination of an object, giving scientists a view of each material’s properties from the atomic perspective without damaging the material.
For more information on the SNS and PPU project, which are supported by the DOE Office of Science, visit http://neutrons.ornl.gov .
This article originally appeared on Oakridger: Planned doubling of SNS power expected to spur scientific discoveries
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